1. Field of the Invention
This invention relates to a power controller for heat tracing cable or other types of electrical resistance heaters which regulates the amount of energy that is input to the load. For example, the controller may be used to maintain a process temperature in situations where liquids are to be prevented from freezing.
2. Description of Related Art
Electrical heat tracing cables are presently used in commercial and industrial markets to provide clean, reliable sources of heat for protecting chemical products and processes. These heaters may be found, for example, on storage vessels and pipe lines, where they are used to heat chemicals waiting transfer from one process function to another. That is, it is often necessary to heat a pipe or vessel to prevent freezing of the liquid or to maintain a certain process temperature in an environment where the ambient temperature may fall below the desired process temperature.
Electrical heater cable systems are typically selected on the basis of their heat output, which must meet or exceed the system's expected heat losses. Heat loss is normally calculated using standard formulas which include parameters such as temperature to be maintained, minimum expected ambient temperature, heat loss from wind, thermal insulation type and thickness, and installation area electrical classification. Heat tracing cable for a pipe installation would be chosen to counteract heat loss, calculated as: EQU Heat loss =q .times..DELTA.T
where q is expressed in watts/foot .degree. F and depends on pipe size and insulation type and thickness, and .DELTA.T is the difference between the desired maintenance temperature and expected minimum ambient temperature.
For example, for a 2" pipe size with 1" thick fiberglass insulation, q=0.070 (See, John Malloy, Thermal Insulation). To maintain a 40.degree. F. pipe temperature (maintenance temperature) in a -20.degree. F. environment (expected minimum ambient temperature) EQU Heat Loss =(0.070).times.(60.degree.) =4.2 watts/foot.
Since 4.2 watts/foot is not a standard heater cable product, the designer might specify 6 watt/foot cable for the installation (giving protection to a minimum temperature of about -46.degree. F.).
There are two types of electric heater cables in widespread use: constant output heaters and self-regulating heaters. The constant output heater cables produce the same amount of heat once they are triggered, regardless of the environment temperature. Heat is supplied from constant output cables for all ambient temperatures below the maintenance temperature. Typically, a thermostat initiates full rated power to constant output cables whenever the ambient temperature is below the maintenance temperature. Constant output cables receive their full rated output (e.g. 6 watts per foot) even if the ambient temperature is only slightly below the maintenance temperature. It follows that electrical energy is wasted by constant output heater cables as more heat is supplied than the environmental conditions require. In the above example, 6 watt/foot constant output cable could nominally maintain a 40.degree. F. maintenance temperature down to about a 46.degree. F. ambient temperature.
Self-regulating heater cables have been proposed as the answer to the wasted energy problem of constant output heater cables and as a prevention to overheating. See, U.S. Pat. No. No. 4,072,848. The self-regulatory cables are promoted on the basis that they supply only about the amount of heat required for actual heat loss. These cables adjust their output according to expansion and contraction of a semiconductive heat producing core. The semiconductor core contains electrically conductive particles (usually graphite) surrounded by heat sensitive plastic which permits or hinders electrical conduction. However, the self-regulating heaters must be energized at 50.degree. F. for freeze protection systems to allow a sufficient response time for rapid changes in ambient temperature. Much higher activation temperatures may be required to provide sufficient heat if a higher process temperature is to be maintained. Additionally, the self-regulating semiconductor cores are very difficult to produce in large quantities. It has been the general practice to make several models of self-regulating heating cable each with a narrow range of energy output. Consequently, oversize self-regulating heating cable must often be used, which results in wasted energy and inefficiency.
The conventional method of controlling vast quantities of electric heater cable circuits for freeze protection is to distribute the power through a power distribution control center energized by an electro-magnetic contactor. A thermostat senses the ambient air temperature and energizes the heater cables at some triggering temperature such as 40.degree. F. or 50.degree. F. See e.g., U.S. Pat. No. 4,575,617. Thus, 100% of maximum rated power for the cable distribution network (as determined by the cable resistance) is supplied for any temperature below the triggering thermostat temperature. Constant output cables will draw all of the electricity according to their manufactured watt per foot outputs. Constant output cables have very little inrush current due to their fixed resistances. On the other hand, the self-regulating cables must heat up to find the thermal equilibrium temperature. Large quantities of inrush current lasting for several minutes may be associated with this process.
Still another method of controlling the power distribution is to use a bulb and capillary thermostat or other surface-mounted temperature sensor for determining the actual temperature of the fluid or container. The thermostat bulb or sensor is usually placed directly on the pipe or other equipment to sense the wall temperature (e.g. using a bimetallic strip, RTD, thermocouple, or thermistor). If the fluid temperature (or container temperature) falls below the maintenance temperature power is supplied to the heater cable. In this arrangement, a range or dead band will be set in which heat is supplied to the heater cable before the heat producing core attains thermal equilibrium. For example, the operator might set the thermostat to supply power to the heat tracing cable at 2.degree. F. below the maintenance temperature and to continue the supply of power until the sensor detects 10.degree. F. above the maintenance temperature. Whether the heat tracing cable is a constant output or self-regulating, this system is using more energy than necessary or desirable. Additionally, this configuration is problematic in that a number of sensors must be accurately placed on the pipe or vessel inside the insulation, which is sometimes difficult and inaccurate.
Therefore, it would be a significant advance in the art if a relatively simple apparatus and method were devised which regulated the power to heat tracing cable in a safe, energy efficient manner. That is, it would be advantageous if such a controller could assure maintenance temperature while supplying only the minimum necessary electrical power.